Polymer electrolyte fuel cells (PEFCs) have the potential of establishing a higher energy density than secondary batteries and are attractive as an energy source of the next generation for portable information terminals and the like. In general, fuel cells use hydrogen (H2) as the fuel. Referring to FIG. 10, anodic reaction takes place to generate protons and electrons, as shown by formula (1), on the side of an anode “a” disposed on one surface of a solid polymer electrolyte “s”. The protons traverse the electrolyte. The electrons move through an external circuit to a cathode “c” where cathodic reaction takes place to reduce oxygen as shown by formula (2). The overall reaction is represented by formula (3), that is, water forms from hydrogen and oxygen.H2→2H++e−  (1)1/2O2+2H++2e−→H2O  (2)H2+1/2O2→H2O  (3)
As the proton donor or fuel, for example, methanol may be used. The fuel cell using methanol is generally known as direct methanol fuel cell (DMFC). As shown in FIG. 11, the DMFC is constructed such that anodic reaction takes place on the side of anode “a” disposed on one surface of a solid polymer electrolyte “s” whereby carbon dioxide is formed from one molecule of methanol and one molecule of water as shown by formula (4), with protons and electrons being concomitantly available. On the side of cathode “c”, cathodic reaction takes place to reduce oxygen as shown by formula (5). The overall reaction is represented by formula (6), that is, water forms from methanol and oxygen.CH3OH+H2O→CO2+6H++6e−  (4)3/2O2+6H++6e−→3H2O(5)CH3OH+3/2O2→CO2+2H2O  (6)
With respect to the oxidant subject to cathodic reaction, gaseous oxygen is ordinarily used as mentioned above, while a supply of oxygen in the form of an oxygen dissolved solution is under investigation for use in compact-size fuel cells which are believed promising as the power supply to portable electric appliances and the like. However, a supply of oxygen saturated solution fails to feed a sufficient amount of oxygen needed by the increasing demand for greater power generation. The oxygen feed to the cathodic reaction becomes a rate controlling factor, failing to provide a satisfactory power generation capability.
When an oxygen dissolved solution having microscopic bubbles of oxygen or air dispersed therein is used for increasing the oxygen feed, problems arise in compact-size fuel cells in which the oxygen dissolved solution must pass through narrow channels. Since the channels are significantly narrow, bubbles can stagnate to clog the channel or disturb the liquid flow. This prevents transportation of protons through the solid polymer electrolyte and smooth progress of cathodic reaction.
Therefore, in connection with fuel cells of the liquid oxidant feed type which are regarded promising among compact-size fuel cells, it would be desirable to have a fuel cell capable of feeding the oxidant to the cathode at a higher efficiency than the oxidant feed capacity of conventional saturated oxygen dissolved solution.
Reference should be made of JP-A 2004-172075, JP-A 2004-165142, and JP-A 2005-197188.